Field of the Invention
Embodiments presented in this disclosure generally relate to varying the speed of a rotor in a wind turbine when operating in a low power mode. More specifically, embodiments disclosed herein vary the rotor speed while operating in a low power mode based on a changing wind speed.
Description of the Related Art
Grid compliance in some countries requires wind turbines to quickly ramp from a low (de-rated) output power to full production (i.e., a wind-optimal output power). For example, a grid code may require the turbines in a wind park to operate in a low power mode upon request so that the turbines are de-rated to output a fraction of the optimal output power for a given wind speed. But when output power decreases, the torque on the rotor also decreases assuming the rotational speed is maintained. As the torque on the rotor decreases, the risk that smearing in the drive train (when ball bearings begin to slide rather than rotate) will occur also increases which can damage the turbine. Moreover, when torque is low, the turbine may generate negative torque which can harm the gear box and significantly reduce its lifespan. To reduce the likelihood of smearing and torque reversals, the rotor speed is decreased when operating in the low power mode thereby preventing the average torque from falling in a range where the drive train or gear box may be damaged.
In addition to requiring the turbines to be capable of operating in a low power mode, some grid codes require that turbines can be ramped back to full production within a short time—e.g., 15 seconds. However, current turbine may be unable to achieve these fast ramp times.